1. Field of the Invention
The present invention relates to a phenanthroline compound and a light-emitting device using the compound. More specifically, the present invention relates to a phenanthroline compound having a molecular structure represented by the following general formula (I) and an organic electroluminescent (EL) device using the compound for a host of a light-emitting layer.
2. Related Background Art
In an old example of an organic light-emitting device, a voltage has been applied to an anthracene evaporated film to emit light (Thin Solid Films, 94 (1982), 171). However, because of ease of large-area production compared with inorganic light-emitting devices, obtainability of desired color emission by the development of various new materials, and low voltage drivability, in recent years, active research including material development is being conducted for the development of an organic light-emitting device as a light-emitting device having high-speed responsibility and high efficiency.
For example, as detailed in Macromol. Symp. 125, 1-48 (1997), an organic EL device is generally structured to have two (upper and lower) electrodes formed on a transparent substrate and an organic substance layer including a light-emitting layer formed between the electrodes. In addition, investigation has been recently made into a device using not only conventional light emission utilizing fluorescence upon transition from singlet exciton to ground state but also phosphorescence via triplet exciton as typified by D. F. O'Brien et al, “Improved energy transfer in electrophosphorescent device”, Applied Physics Letters, Vol. 74, No. 3, p. 422 (1999) and M. A. Baldo et al, “Very high-efficiency green organic light-emitting devices based on electrophosphorescence”, Applied Physics Letters, Vol. 75, No. 1, p. 4 (1999). In these articles, an organic layer having a four-layer structure is mainly used. The structure is composed of a hole-transporting layer, a light-emitting layer, an exciton diffusion-prevention layer, and an electron-transporting layer stacked in the mentioned order from an anode side. The materials used are carrier transporting materials and a phosphorescent material Ir(ppy)3 shown below.

Further, emission of a light from ultraviolet to infrared region can be performed by changing the kind of a fluorescent organic compound. In these days, research has been actively conducted on various compounds.
In addition to organic light-emitting devices using such low-molecular materials as those described above, a group of the University of Cambridge has reported organic light-emitting devices using conjugate polymers (Nature, 347, 539 (1990)). This report has confirmed that light emission can be obtained by a single layer by forming polyphenylene vinylene (PPV) in a film shape by use of a coating system.
As described above, recent progress of an organic light-emitting device is remarkable, and is characterized in that a highly responsive, thin, and lightweight light-emitting device that can be driven at a low applied voltage and provides a high luminance and a variety of emission wavelengths can be made, which suggests the applicability to a wide variety of uses.
However, at present, an optical output of a higher luminance and a higher conversion efficiency have been required. In addition, there still remain a large number of problems in terms of durability such as a change over time during long-term use and degradation due to an atmospheric gas containing oxygen or to moisture. Furthermore, light emission of blue, green and red colors having a high color purity is necessary when application to a full-color display or the like is attempted. However, those problems have not been sufficiently solved yet.
In addition, a large number of aromatic compounds and condensed polycyclic aromatic compounds have been studied as fluorescent organic compounds used for an electron-transporting layer, a light-emitting layer, and the like. However, it is difficult to say that a compound sufficiently satisfying the emission luminance and durability requirements has been already obtained.
Examples of patent documents describing application of a phenanthroline compound to an organic EL, which is related to the present invention, include Japanese Patent Application Laid-Open Nos. H05-331459, 2001-267080 and 2001-131174. However, none of these patent documents discloses an organic compound in accordance with the present invention characterized by including a partial structure containing a fluorene ring and a phenyl ring on a straight line in a molecular structure.
In order to apply an organic EL device to a display unit of a display apparatus or the like, the device is required to have an optical output of a high efficiency and a high luminance and sufficiently secure high durability. However, such requirement has not been sufficiently met.